1. Field of Invention
This invention relates to a device for radiation treatment for cancer and the method thereof. It more specifically relates to a device to generate helical electron beams for radiation therapy for tumors and the method of irradiating tumors with a helical electron beam.
2. Description of the Related Art
Radiation therapy has been effective for managing early stage (stage I-II) breast cancers following breast conservation surgery (BCS). More recent studies have indicated that radiation therapy also improves survival rate for late stage (stage III-IV) breast cancer patients. However, the high risk of cardiac or pulmonary complications often compromises the treatment of these patients using conventional radiation therapy techniques.
Conventional photon beams have been used successfully for treating early stage breast cancer treatment. It is now widely accepted that radiation therapy following breast conservation therapy is as effective as mastectomy for early breast cancers [1-9]. Radiation therapy has also been used for managing late-stage breast cancer patients [10-16]. More recent studies have found that radiation therapy improves survival for a subgroup of patients with late-stage breast cancer [15-16].
Conventional radiation therapy techniques use two opposing tangential photon beams to treat the whole breast. The advantage of photon beam is that it has sharp beam delineation and high penetrating power. The disadvantages of photon beam treatment are as follows:
(1) Limited beam setup options. Patient geometry may sometimes include excessive heart or lung volume for irradiation.
(2) Unable to treat internal mammary nodes because of the proximity of the node chains to the heart.
(3) Potential high scatter dose to the contralateral breast.
(4) Patient breathing motion may introduce accidental irradiation of large volume of heart or lung.
These restrictions especially the first one have restricted the use of photon beam radiation therapy for advanced breast cancer treatment. Several clinical studies have reported significant rate of cardiac or pulmonary toxicity associated with the conventional photon beam techniques [17-27]. High risk of irradiating the heart or lung becomes critical for post-mastectomy patients when they receive chest wall or internal mammary node treatments [20-21]. The limitation of photon beam breast treatment is evident from a recent study that compares the photon Intensity Modulation Radiotherapy (IMRT) technique with the conventional radiation therapy techniques for breast cancer treatment [28]. The results of the study show that IMRT presents almost no advantage over the conventional techniques for whole breast treatment. This result implies that there may be not much room in improving the existing techniques for photon beam treatment of breast cancers.
Electron beams, on the other hand, offer attractive features for breast cancer treatment. First, breast tumors are relatively shallow that could be penetrated by electrons of 18-20 MeV. These electron beams are already available for most medical accelerators. Second, the electron beam exhibits fast fall-off near the end of its range of travel. These features enable treatment of the internal mammary nodes or the chest wall region while sparing the heart or lung underneath. Because the anterior-posterior direction of the breathing motion coincides with the beam-on direction, the simple treatment setup reduces the possibility of over irradiation of the heart or lung due to patient breathing motion. This is analogous to the treatment of lung cancer patients where direct anterior-posterior setup has been found to be the most reliable and effective treatment method [29]. In comparison, electron beam would overcome all the above listed disadvantages associated with the photon beam treatment. One major disadvantage of electron beams is their excessive scatter and beam penumbra.
Recent rapid developments in computer controlled beam delivery systems have become a driving force in developing intensity modulation radiotherapy (IMRT) using high-energy photon beam [30-49]. The technology has become mature enough that several clinical trials are near their completion [48-49].
Because of the high cost and few locations that can handle high-energy photon beam radiation therapy, it would be useful to have a different approach to reduce radiation to non-cancerous tissue while increasing the radiation to cancerous tissue. One approach would be to use electron beams. But the problem with electron beams is their tendency to scatter.
If one could reduce the problems associated with electron beam radiation, one would be able to develop intensity modulated electron beam for Avoidance Radiation Therapy (ART). The idea of ART has been applied to almost every aspect of radiation therapy where a tumor dose is typically limited by the tolerance of normal tissue structures. If normal tissue dose could be significantly reduced or avoided, then the complication rate would be reduced. This reduction in radiation of normal tissue would permit dose escalation to achieve better local control of the tumor and improves the prospects for patient survival [50]. Several promising beam modulation techniques have been proposed for ART treatment using electron beams [51-57].
One approach is using finely collimated beams scanning across the entire treatment area using more sophisticated beam transport systems such as those used in the Racetrack Microtron [51-54]. This approach requires significant modification of the design for most clinical linear accelerators; and thus is not feasible. Another approach is to simply use the photon beam multileaf collimator system (MLC) for electron-beam intensity modulation [57]. This approach has not yet solved the problem the excessive electron beam scatter effects.
There have been several theoretical studies and preliminary experiments exploring the use of static magnetic fields in shaping clinical electron beams [58-63]. None of these studies and experiments has proven to be successful yet.
Several unique beam delivery algorithms using computer controlled MLC systems have been recently developed for beam delivery optimization and verification [39-47]. Conventional three-dimensional pencil beam algorithm is a fast and practical method used in most clinical treatment planning systems [64-65]. A more time-consuming Monte Carlo method has also been recently developed particularly for various heterogeneous cases [66-68]. These algorithms could be used or modified for an improved electron beam radiation system.